Many of us are familiar with platoon effects in baseball — batters tend to hit better against pitchers of the opposite side. But how has the platoon effect impacted the hand of pitchers and hitters in baseball history? Also, how often do pitchers have the platoon advantage (pitching against batters of the same hand), and how does this advantage change across innings? We construct several graphs that show how handedness and platoon advantage have changed across baseball history.

**Batter and Pitcher Hand**

Let’s look first at the hand of the batter and the pitcher. I collected Retrosheet play-by-play data for the seasons 1964, 1974, 1984, 1994, 2004, and 2014. For season and each inning, I computed the proportion of right-hand pitchers and right-hand batters among all plate appearances. Here’s a graph that shows how these right-handed proportions have changed across seasons and innings.

Here are some insights from this graph.

- Generally, the proportion of right-handed pitchers has slowly increased over seasons. In the early seasons (1964, 1974, 1984), the percentage of right-handers was in the high 60’s and it has increased to the low 70’s in recent seasons.
- There is a right-handed closer effect. In late innings, the pitchers are more likely to be right-handed. This effect is especially strong in recent seasons — in 2004, over 80% of the 9th inning pitchers are right-handed and the proportion of 9th inning righties has slightly decreased in 2014.
- In contrast, the proportion of right-handed batters has shown a steady decrease across seasons.
- The proportion of right-handed batters also tends to decrease across innings. The smallest proportion of righties (largest proportion of lefties) happens in the 9th inning.
- There is a funny 1st-2nd inning effect. Right-handed hitters are much more common in the 2nd inning opposed to the 1st inning.

**Platoon Advantage**

Okay, we’ve seen some interesting patterns about the hand of pitchers and batters. What about the platoon advantage? For each season and each inning, I compute (for both left and right handed pitchers), the proportion of plate-appearances where the pitcher had a platoon advantage (pitching against a batter of the same side). Here’s the graph.

What do we see?

- For right-handed pitchers, the platoon advantage has generally decreased over seasons. In 1964, roughly 60% of the time the righty had a platoon advantage and it has decreased to 50% in recent seasons.
- For righties, the platoon advantage stays pretty steady across innings with a couple of exceptions. First, the platoon advantage is small in the first inning, and it is greatest for late innings.
- Left-handed pitchers are less likely than righties to have a platoon effect.
- For early seasons, the leftie platoon advantage generally increases across innings. In recent seasons, the leftie advantage dramatically increases over innings. Clearly, there is much more substitution of lefties in last innings to take advantage of the platoon advantage.
- In recent seasons, there is a 9th inning drop off in the platoon advantage for lefties — it appears that closers tend to be right-handed.

These figures address baseball strategy regarding platooning. Managers clearly believe it is a significant factor and it plays a role in managing practically all games. These graphs say nothing about the actual size of the platoon effect. For example, it would be interesting to see how platoon strategy has addressed run scoring, and it would be interesting to find players who possess abilities to do well against pitchers of the same hand.

By the way, all of the R code to create these graphs for a particular season can be found here.